Would the percent of oxygen be more or less if any of the following

 
Would the percent of oxygen be more or less if any of the following happened? Give a reasonable explanation of your answer.
(a) The magnesium was heated twice but not to a constant weight.
(b) The crucible lid was left on the crucible during the entire heating.
(c) The crucible lid was left off during the heating and the magnesium was so hot it started to react with the nitrogen in the air as well as the oxygen.

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